Optical spectrum analyzers are used for analyzing the output light beams from lasers, light-emitting diodes and other light sources. Optical spectrum analyzers are particularly useful for analyzing light sources for optical telecommunication, where it is preferable to ensure that the optical carrier includes only a single, spectrally pure wavelength. In optical spectrum analyzers, the light intensity of a light beam is displayed as a function of wavelength over a predetermined wavelength range. Parameters of importance in optical spectrum analyzers include wavelength range, wavelength and amplitude accuracy, sensitivity, resolution, measurement speed, polarization insensitivity and dynamic range.
Optical spectrum analyzers frequently use a diffraction grating for separating the light beam to be analyzed into its component wavelengths. The input light beam to be analyzed is collimated and is directed at the diffraction grating. The light beam is spatially dispersed by the grating, since different wavelengths are diffracted at different angles. The grating is rotated so that the dispersed light beam is scanned over a slit. The light that passes through the slit is detected to provide an output signal that represents amplitude as a function of wavelength. The width of the slit establishes the resolution of the monochromator. Prior art optical spectrum analyzers have used single stage monochromators as described above and two stage monochromators, wherein two monochromators operating in series provide improved performance. The output electrical signal is displayed as a function of diffraction grating position to provide an optical spectrum of the input light beam.
Optical spectrum analyzers should operate over a wide range of parameter values for maximum versatility. For example, the selected wavelength span may range from a few nanometers to a few hundred nanometers. In some cases, the user may wish to accurately measure low amplitude signals, thereby requiring long sweep times. In other cases, accurate measurement of small amplitude signals may not be important to the user, and the sweep rate can be increased. Prior art optical spectrum analyzers have not adequately addressed the wide variety of user requirements.
Optical spectrum analyzers with optical fiber inputs sometimes receive low power signal levels, which require long sweep times. Long sweep times are required because of the need for averaging to increase the signal-to-noise ratio. Frequently, the user selects a reference level to capture a peak of a signal and specifies a vertical amplitude scale on the optical spectrum analyzer display. However, without any other information, the instrument may be required to sweep very slowly in order to accurately display signals of the power level represented by the bottom of the display screen. The long sweep times are not acceptable to the user when the signal level represented by the bottom of the display is not of interest.
Prior art optical spectrum analyzers usually require the user to select a detection or averaging method, or a mode where the instrument is either highly sensitive or less sensitive. Prior art systems have had several disadvantages. The user is not provided with information as to the minimum signal that is accurately displayed by the instrument. Furthermore, by providing only two or three instrument modes, the instrument is in most cases sweeping more slowly than necessary for the minimum detectable signal that is to be displayed. In addition, prior art systems have typically not provided information as to how the measurement is being taken. Without such information, erroneous measurements can occur.
It is a general object of the present invention to provide improved methods and apparatus for optical spectrum analysis.
It is another object of the present invention to provide methods and apparatus for optical spectrum analysis wherein the sensitivity is adjustable by the user.
It is another object of the present invention to provide methods and apparatus for optical spectrum analysis wherein peak detectors are incorporated in the video channel to accurately measure signal amplitudes in a fast scanning condition.
It is a further object of the present invention to provide methods and apparatus for optical spectrum analysis wherein the sweep time is optimized for a given set of measurement parameters.
It is yet another object of the present invention to provide methods and apparatus for optical spectrum analysis wherein the sweep time and video bandwidth are automatically adjusted in response to user selection of sensitivity.
It is still another object of the invention to provide methods and apparatus for optical spectrum analysis wherein the sensitivity is displayed to the user.